A printer which performs printing on a paper sheet has a paper sheet sensor which optically detects a paper sheet. This paper sheet sensor includes a light emitting element and a light receiving element facing each other across a paper sheet carrying path. An output voltage of the light receiving element changes in accordance with whether light emitted from the light emitting element reaches the light receiving element or not.
That is, if there is no paper sheet between the light emitting element and the light receiving element, light from the light emitting element reaches the light receiving element and the output voltage of the light receiving element turns to a light receiving level. When a paper sheet enters between the light emitting element and the light receiving element, light from the light emitting element is interrupted by the paper sheet and does not reach the light receiving element. Thus, the output voltage of the light receiving element turns to a non-light receiving level.
For example, JP-A-2008-13289 discloses an apparatus having such a paper sheet sensor.
Detection targets of the paper sheet sensor include papers of various paper qualities and thicknesses such as thermal sheet, passbook, and normal paper. Depending on paper quality and thickness, light from the light emitting element may be transmitted through the paper sheet and reach the light receiving element. In such case, the output voltage of the light receiving element turns to the light receiving level despite the presence of the paper sheet between the light emitting element and the light receiving element. Thus, a detection error occurs indicating that there is no paper sheet.